vi: code shrink; save/restore errno in signal handlers

vi: code shrink; save/restore errno in signal handlers

function                                             old     new   delta
query_screen_dimensions                                -      54     +54
suspend_sig                                           50      64     +14
cont_sig                                              65      66      +1
catch_sig                                             42      32     -10
winch_sig                                             88      60     -28
edit_file                                            719     671     -48
refresh                                              848     767     -81
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 2/4 up/down: 69/-167)           Total: -98 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>